Turbomolecular vacuum pump

ABSTRACT

A turbomolecular pump comprises a pumping mechanism and an electric motor ( 10 ) for driving the pumping mechanism. The electric motor comprises a rotor ( 14 ), a stator and stator windings ( 20 ). The stator comprises a yoke [ 18 ( 1 ),  18 ( 2 )] and a plurality of teeth ( 16 ) projecting from the yoke and carrying the stator windings. The yoke and the teeth are made of a non-sintered magnetic powder material in which the powder particles are bonded by an electrically insulating binder.

The invention relates to turbomolecular vacuum pumps and to high-speedelectric motors for driving turbomolecular vacuum pumps.

High-speed motors for turbomolecular vacuum pumps must be capable ofoperating at very high speeds in difficult operating conditions.Although the motor speed may be as low as 20,000 rpm, typically, themotor speed will be in excess of 50,000 rpm and may be up to 100,000rpm.

An electric motor driving a turbomolecular pump is mounted within thepump casing in an area which experiences high vacuum. This gives rise todifficulties in cooling the rotor since the sole heat path forconduction of the heat generated when in use, is through the bearings ofthe shaft carrying the rotor and only a small amount of heat can bedissipated by radiation into the process gas. For this reason, brushlesspermanent magnet motors are usually used. In order to reduce eddycurrent losses and smooth the output of such motors, their statorsconventionally have six or more teeth with distributed windings. Thesemotors are specifically designed for application in turbomolecularvacuum pumps and their relatively complicated structure makes themexpensive to produce, especially as the volumes required for use withturbomolecular pumps are relatively low. In particular, forming thewindings in the limited spaces available within conventional statordesigns is a difficult and expensive process involving dedicatedmachinery.

An important consideration in the design of motors for turbomolecularpumps has been the need to minimise the thickness of the rotor magnet soas to maintain a relatively large diameter shaft to provide thestiffness necessary for high-speed rotation.

A further problem with motors for turbomolecular pumps is outgassingfrom the components of the motor. It is known that gas becomes lodged onor below the surfaces of the pump components and when the surfaces areplaced under vacuum, gas evolves from those surfaces. This generation ofgas by desorbtion is known as known outgassing. Outgassing is anincreasingly important factor when pumps work at higher levels of vacuumand it is desirable to reduce outgassing levels to the extent this maybe possible.

It is an object of the invention to provide an electric motor suitablefor use in a turbomolecular pump that is more economical to produce thanconventional motors and/or provide improved outgassing performancecompared with conventional motors and/or at least provide an alternativechoice of motor design.

The present invention provides a turbomolecular vacuum pump comprising apumping mechanism and an electric motor for driving the pumpingmechanism, the electric motor comprising a rotor, a stator and statorwindings, the stator comprising a yoke and a plurality of teethprojecting from the yoke and carrying the stator windings, the yoke andthe teeth being made of a non-sintered magnetic powder material in whichthe powder particles are bonded by an electrically insulating binder.

The invention also includes an electric motor for a turbomolecularvacuum pump, the motor comprising a stator having a plurality of teeththat have respective radially innermost surfaces that define an insidediameter of the stator, a motor shaft and a rotor comprising a permanentmagnet mounted on a portion of the motor shaft, wherein a ratio of anoutside diameter of the motor shaft portion and the inside diameter ofthe stator is less than or substantially equal to 2:3.

The invention also includes an electric motor for a turbomolecularvacuum pump, the motor comprising a stator having a plurality of teeththat each have a curved radially innermost surface and are arranged soas to define a substantially circular through-passage, a motor shaft anda rotor disposed in the through-passage and mounted on the motor shaft,the rotor having an outside diameter and comprising a permanent magnetthat defines a through hole by which the rotor is mounted on the motorshaft and wherein the through hole defines an inside diameter of therotor and a ratio of the inside diameter of the rotor and the outsidediameter of the rotor is less than or substantially equal to 2:3.

The invention also includes an electric motor for a turbomolecular pump,the motor comprising a stator, a rotor and a motor shaft, the statorcomprising three teeth made of a powdered magnetic material and eachcarrying an electrical winding, the teeth having concave radiallyinnermost surfaces and being arranged such that the concave surfacesdefine a substantially circular passage for the rotor, the passagehaving a diameter, the rotor being mounted directly on the motor shaftand comprising a core made of a polymer bonded magnetic material anddefining an axially extending through-passage for the motor shaft, therotor further comprising a reinforcing sleeve for the core, the rotorbeing mounted on the motor shaft, which motor shaft has a diameter, andwherein a ratio of the motor shaft diameter to the passage diameter isless than or substantially equal to 2:3.

The invention also includes a turbomolecular vacuum pump comprising acasing having a pump inlet and a pump outlet, a pumping mechanism housedin the casing and connected with the inlet and the outlet and anelectric motor housed in the casing and connected with the pumpingmechanism, the electric motor comprising a motor shaft having adiameter, a rotor mounted directly on the motor shaft and consisting ofan annular core and a reinforcing sleeve for the core, the core being apolymer bonded magnet and the sleeve being made of a material having lowelectrical conductivity, and a stator comprising three teeth held in atwo-piece yoke and each carrying a wound non-distributed electricalwinding, the teeth being made from a magnetic powder material and eachcomprising an arcuate portion having a concave surface and a convexsurface opposite the concave surface and a projection projectingradially from the convex surface, the concave surfaces being arranged todefine a substantially circular passage having an axis and a diameterand the rotor and motor shaft being coaxially disposed in the passage,wherein a ratio of the motor shaft diameter to the passage diameter isless than or equal to 2:3.

The invention also includes an electric motor for a turbomolecular pump,the motor comprising a stator having a plurality of teeth for carryingrespective electrical windings, a motor shaft comprising a hollow membermade of a material having low electrical conductivity and a rotorcomprising a permanent magnet housed in the hollow member.

The invention also includes an electric motor for a turbomolecular pump,the motor comprising a stator having a plurality of teeth for carryingthe motor windings and a tooth holding unit by which the teeth aresupported, the teeth and the tooth holding unit being made of a magneticpowder material.

In order that the invention may be well understood, an embodimentthereof, which is given by way of example only, will now be describedwith reference to the drawings, in which:

FIG. 1 is a schematic representation of a turbomolecular pump;

FIG. 2 is an exploded perspective view of a motor for a turbomolecularpump;

FIG. 3 is a perspective view of the motor in an assembled condition withthe motor shaft omitted; and

FIG. 4 is a section on line IV-IV in FIG. 2.

FIG. 1 shows a turbomolecular vacuum pump 2 that comprises a casing 4having an inlet 5 and an outlet 6. The casing 4 houses a pumpingmechanism 8 connected with the inlet 5 and the outlet 6 and an electricmotor 10 for driving the pumping mechanism 8. Pumping mechanisms forturbomolecular vacuum pumps are know per se and will therefore not bedescribed in any detail herein. The pumping mechanism could, forexample, be a mechanism such as that in the EXT250 made by BOC Edwardsof the United Kingdom.

Referring to FIG. 2, the motor 10 comprises a motor shaft 12, a rotor14, a stator and stator windings 20. The stator comprises three teeth 16and a two-piece-split yoke 18(1), 18(2).

The teeth 16 are pressed, non-sintered, components made of soft magneticpowder, in which the powder particles are bonded by an electricallyinsulating binder. An example of a suitable powder material providedwith such a binder is Somaloy 500 made by Hoganas of Sweden. It is to benoted that although heat may be applied during the pressing process, itshould be insufficient to cause fusion of the particles, which are heldtogether by the binder.

Each tooth 16 comprises an arcuate segment 22 and a stepped projection24 projecting from the convex side 26 of the arcuate segment. Thestepped projections 24 provide a locating point for the windings 20 inthe form of a first portion 28 that is shaped such that it can bereceived in an elongate slot 30 defined by the windings 20. The firstportion 28 of the projection 24 is positioned on and extends radiallyoutwardly from the convex side 26 of the arcuate segment 22. A secondportion 32 extends radially outwardly from the first portion 28. Thesecond portion 32 is generally circular and smaller in cross-sectionthan the first portion 28.

The two-piece split yoke comprises two annular members 18(1), 18(2).Like the teeth 16, the annular members 18(1), 18(2) are pressed,non-sintered components made of soft magnetic powder, in which thepowder particles are bonded by an electrically insulating binder. Eachannular member 18(1), 18(2) has three semi-circular cut-outs 34 at itsaxially inner end. The cut-outs 34 are equi-spaced about thecircumference of the annular members 18(1), 18(2) at 120° intervals. Thearrangement is such that when the innermost side faces 36 of the annularmembers are brought into abutting relationship, three circular holes 40(FIG. 2) are provided for receiving the second portions 32 of thestepped projections 24. It will be appreciated that the second portions32 and the holes 40 are circular as a matter of convenience and thatother shapes could be used if desired.

As shown in FIG. 4, when the motor 10 is assembled, the second, freeend, portions 32 of the stepped projections 24 are held in therespective holes 40 defined by the opposed cutouts 34. The teeth 16 areheld such that the concave radially innermost surfaces 42 of the arcuatesegments form a substantially continuous circular passage extendingaxially through the stator. The passage defined by the concave surfaces42 has a diameter corresponding to the radial distance R of the surfacesfrom the longitudinal axis of the stator and this diameter can beconsidered the inside diameter of the stator.

The electrical windings 20 are formed by entirely separate coils ofwound wire that may be wound onto bobbins made of plastic to provideelectrical insulation. The windings each have tails (not shown) by whichthey are connected to an a.c. electrical supply.

When the motor is assembled, the stator parts (the annular members18(1), 18(2) and teeth 16) are preferably bonded as a unit using asuitable adhesive. This sub-assembly is then assembled into the pump andpreferably then set in resin. However, if desired, other assemblytechniques can be used.

The rotor 14 comprises an annular core 44 made of a magnetic materialhaving low electrical conductivity. The core 44 may be made of ferrite,but in a presently preferred embodiment is made of a polymer bondedmagnetic material. Polymer bonded magnetic materials are composites ofnon-conductive polymer and embedded magnetic particles. Such materialswill be well known to those skilled in the art and will not be describeddetail herein. An example of a suitable material is Vacobond (TradeName) made by Vacuumschmelze of Hanau, Germany. The core 44 has anaxially extending through-hole 46 for receiving the motor shaft 12. Therotor 14 additionally comprises a sleeve 48 for the core 44. The sleeveserves as a containing, or reinforcing, member for the rotor and may bemade of any suitable material. The sleeve is made of a material havinglow electrical conductivity. Preferably, in practical terms, thematerial is electrically non-conductive. In a preferred embodiment, thesleeve 48 is thin and made of a carbon-fibre reinforced plastic (CFRP).The sleeve need only be sufficiently thick as to provide the desiredreinforcing and it is preferable that it is kept as thin as ispracticable.

The diameter of the through-hole 46 essentially corresponds to theoutside diameter of the motor shaft 12 so that the core 44 is a closesliding fit on the motor shaft and can be securely fixed thereon. Themotor shaft diameter is selected such that the ratio of the insidediameter of the stator and the outside diameter of the motor shaft, orat least the portion of the shaft on which the rotor 14 is mounted, isnot greater than 3:2. In an embodiment of the motor 10, the shaftdiameter is 14 mm and the outside diameter of the rotor 14 is 21 mm.Since there is just a very small running clearance between the surfaces42 of the teeth 16 and the outer surface of the sleeve 48 (exaggeratedin FIG. 3), the inside diameter of the stator corresponds substantiallyto the outside diameter of the rotor. Accordingly, when the outsidediameter of the rotor is 21 mm and the shaft diameter is 14 mm, thisessentially corresponds to a 2:3 ratio of shaft to stator insidediameter. In another embodiment the motor shaft diameter is 13.7 mm,while in yet another embodiment, the motor shaft diameter is 10 mm, withthe stator inside diameter being 21 mm in each case. In the examplesgiven above, the sleeve thickness will be approximately 1 mm and ispreferably as thin as is possible within the constraints of providingthe reinforcing desired.

By reducing the diameter of the motor shaft as much as possible withinthe allowable limits of required stiffness, it is possible to provide adeeper magnetic core 46 (by deeper it is meant that the thickness of thecore in the radial direction is increased). Increasing the distancebetween the stator and motor shaft in this way serves to minimise therotor losses arising from high-speed operation of the motor. If suchlosses are not minimised, significant heating of the shaft can occur. Inthe vacuum environment found within a turbomolecular pump, it is verydifficult to cool the motor shaft. A small amount of heat can bedissipated via the process gases passing through the pump, but the onlyother heat path is through the bearings. The consequent temperaturedifference imposed on the bearings is detrimental to them and results ina reduction of their useful life. By minimising the losses andtherefore, the heating of the shaft, bearing life can be extendedthereby potentially reducing the servicing requirement for the pump.

The use of a deeper polymer bonded magnet together with theconfiguration and relative positioning of the other components in thedescribed embodiment provides the advantage of minimising eddy currentlosses found in the motor shaft arising from the space harmonicsassociated with the three tooth configuration of the motor. By using athree tooth configuration, instead of six or more teeth, as isconventional in high speed motors for turbomolecular vacuum pumps, themotor design is greatly simplified making it easier and more economic toproduce.

It is envisaged that a triangular plate 49 (FIG. 3) may be fitted to anend of the two-piece yoke 18(1), 18(2) by means of screws or the like.The triangular plate would be provided with a through-hole for the motorshaft 12 and carry a sensor arranged to detect the rotational positionof the shaft 12. The sensor would provide signals for use in controllingthe switching of the electrical windings 20. The sensor may, forexample, be a Hall sensor ring. It will, however, be understood thatother sensors could be used and it is not essential that the sensor iscarried by the two-piece split yoke 18(1), 18(2). One alternative thatcould be used if the electrical windings 20 are wound onto bobbins madeof plastic, would be to provide one or more of the bobbins with aprojection by means of which as sensor, such as a Hall sensor ring, canbe supported.

It has been found that a stator consisting of parts made of a magneticpowder material in which the powder particles are bonded by anelectrically insulating binder, is capable of providing the necessaryelectrical properties, while at the same time providing improvements inoutgassing performance. Tests have been carried out in which theoutgassing performance of an embodiment provided with such a stator wascompared with that of a pump having a stator comprising stacked, orlaminated steel plates. It was found that the powdered metal statorprovided a five-fold improvement in outgassing performance. That is, theamount of gas evolving from the stator made of magnetic powder materialwas approximately one-fifth of that evolving from the equivalent statorconstructed as a lamination of steel plates.

It will be appreciated that the modular construction of the stator makesthe motor 10 more economic to produce than conventional motors,particularly if production levels are low, and provides considerabledesign flexibility. For example, forming the teeth 16 as separatecomponents from magnetic powder material allows automated productionusing proven powder technology and the tailoring of the tooth shape toprovide optimum electromagnetic properties. The modular constructionalso allows the electrical windings to be produced as individual coils20 and then when complete, simply positioned on the teeth 16 prior toassembly of the stator. Once the coils are seated on the respectiveteeth, the stator can be readily assembled by clamping the requirednumber of teeth between the annular is members of the two-piece yoke18(1), 18(2). Although it is preferred that a three tooth configurationis used, it will be recognised that the modular construction makes itstraight forward to produce motors having a different number of teeth sothat again the freedom to design a motor having particular performancecharacteristics is enhanced. Thus, when compared with conventionalmotors used in turbomolecular vacuum pumps, the motor 10 has the twinadvantage that it can be produced economically in small volumes and thedesign details can be easily modified to tailor the performancecharacteristics to the particular requirements of a given application.

It will be understood that while the yoke of the embodiment is atwo-piece split unit as shown, other configurations are possible. Forexample, the yoke could be made up of a plurality of arcuate segmentsthat combine to form an annular part. In this case, instead of providingthe cutouts in the axially facing side faces of the segments, as in theembodiment, the recesses for the teeth would be provided between theabutting end surfaces of the segments.

While it is preferred that the stator simply consists of a yoke andteeth made of a non-sintered magnetic powder material in which thepowder particles are bonded by an electrically insulating binder, it ispossible that the parts defining the yoke and teeth may be held in otherparts not made of such a material. For example, in the embodiment, themembers defining the yoke might be clamped in a sleeve made of any othersuitable material. However, this is not preferred since it may reducethe improvement in outgassing performance demonstrated when the statorconsists of components made of the powder material.

In the illustrated embodiment, the sleeve 48 is described as being asthin as practicable within the constraints of providing thereinforcement for the core 44 made of polymer bonded magnetic material.In an alternative embodiment, the sleeve could be extended to serve asthe motor shaft while still housing the magnetic core 44. In this case,the sleeve thickness would be increased to provide the necessary shaftstiffness for high speed motor operation.

In another alternative arrangement instead of having an electrically lowconductive magnet housed in an electrically low conductive sleeve, therotor magnet may be housed in a sleeve made of an electrically highlyconductive sleeve made of a material such as aluminium. In this case, itwould not be essential that the magnet was made of an electrically lowconductive material. With a sleeve made of an electrically highlyconductive material such as aluminium, the stator flux is transmittedinto the sleeve such that the eddy current losses found in the motorshaft are minimised. Since the resistivity of such a highly conductivesleeve will be low, heating of the sleeve component will be low.

1. A turbomolecular vacuum pump comprising a pumping mechanism and anelectric motor for driving the pumping mechanism, the electric motorcomprising a rotor, a stator and stator windings, the stator comprisinga yoke and a plurality of teeth projecting from the yoke and carryingthe stator windings, the yoke and the teeth being made of a non-sinteredmagnetic powder material in which particles in the powder material arebonded by an electrically insulating binder.
 2. The pump according toclaim 1 wherein the stator comprises three of the teeth.
 3. The pumpaccording to claim 1, wherein the stator windings are non-distributedwindings.
 4. The pump according to claim 1 wherein the rotor comprises apermanent magnet.
 5. The pump according to claim 4 wherein the permanentmagnet comprises a material having low electrical conductivity.
 6. Thepump according to claim 5 wherein the permanent magnet is a polymerbonded magnet.
 7. The pump according to claim 6 wherein the rotorcomprises a reinforcing sleeve for the polymer bonded magnet.
 8. Thepump according to claim 7 wherein the sleeve comprises a material havinglow electrical conductivity.
 9. The pump according to claim 8 whereinthe sleeve comprises a carbon reinforced plastics material.
 10. The pumpaccording to claim 4 wherein the rotor further comprises a sleevecomprising an electrically highly conductive material.
 11. (canceled)12. The pump according to claim 1 wherein at least one of the pluralityof teeth has a concave radially innermost surface and a projectionprojecting radially outwardly with respect to a corresponding one of theradially innermost surfaces.
 13. The pump according to claim 12; whereinplurality of yoke members have side surfaces disposed in abuttingrelationship so that a plurality of recesses are formed between the sidesurfaces.
 14. The pump according to claim 13 wherein each of theplurality of yoke members comprises a first annular part having a sidesurface and a second annular part having a side surface wherein therecesses for the projections Beware defined by at least one of the sidesurfaces of the first annular part and the second annular part.
 15. Thepump according to claim 12 further comprising a motor shaft; and whereinthe substantially circular passage defines an inside diameter of thestator, the rotor has an outside diameter and defines a through hole bywhich the rotor is mounted on the motor shaft, the through hole definesan inside diameter of the rotor and wherein the ratio of the outsidediameter of a portion of the motor shaft on which the rotor is mountedto the inside diameter of the stator is less than or substantially equalto 2:3.
 16. The pump according to claim 15 wherein the rotor outsidediameter is less than the stator inside diameter and wherein the insidediameter of the rotor is substantially equal to the outside diameter ofthe portion of the motor shaft on which the rotor is mounted.
 17. Thepump according to claim 12 wherein the substantially circular passagedefines an inside diameter of the stator, the rotor has an outsidediameter and defines a through hole by which the rotor is mounted on amotor shaft and the through hole defines an inside diameter of therotor, wherein the ratio of the inside diameter of the rotor and to theoutside diameter of the rotor is less than or equal to 2:3.
 18. The pumpaccording to claim 1; wherein the electric motor has speed from 20,000to 100,000 rpm.
 19. The pump according to claim 1 wherein at least oneof the plurality of teeth has a concave radially innermost surface and aprojection projecting radially outwardly with respect to a correspondingone of radially innermost surfaces, and wherein the radially innermostsurfaces are arranged to form a substantially circular passage andwherein the yoke is adapted to have recesses for receiving theprojections.